Numerous methods and devices for determining at least one flow property of fluid media, i.e., liquids and/or gases, are available. The flow properties as possible parameters may be any physical and/or chemical measurable properties which qualify or quantify a flow of the fluid medium. In particular, this may be a flow speed and/or a mass flow and/or a volume flow.
The present invention is described in the following in particular with reference to so-called hot-film air-mass meters, as described, for example, from Konrad Reif (eds.): Sensors in the motor vehicle, 1st edition 2010, pages 146-148. Such hot-film air-mass meters are generally based on a sensor chip, in particular a silicon sensor chip, for example, including a sensor diaphragm as measuring surface or sensor area, over which the flowing fluid medium may flow. The sensor chip generally includes at least one heating element and at least two temperature sensors, which are situated, for example, on the measuring surface of the sensor chip, one temperature sensor being mounted upstream from the heating element and the other temperature sensor being mounted downstream from the heating element. From an asymmetry of the temperature profile detected by the temperature sensors, which is influenced by the flow of the fluid medium, a mass flow and/or a volume flow of the fluid medium may be inferred.
Hot-film air mass-meters are usually designed as plug-in sensors, which may be fixedly or exchangeably inserted into a flow tube. For example, this flow tube may be an intake tract of an internal combustion engine.
In this case, a partial flow of the medium flows through at least one main channel provided in the hot-film air-mass meter. A bypass channel is formed between the inlet and the outlet of the main channel. In particular, the bypass channel is designed in such a way that it has a curved section for deflecting the partial flow of the medium which has entered through the inlet of the main channel, whereby the curved section subsequently transitions into a section in which the sensor chip is situated. The last-mentioned section represents the actual measuring channel, in which the sensor chip is situated.
Such hot-film air-mass meters have to satisfy a large number of requirements in practice. In addition to the aim of reducing a pressure drop on the hot-film air-mass meter as a whole using suitable embodiments with respect to flow, one of the main challenges is to further improve the signal quality and the robustness of the devices against contamination by oil and water droplets as well as soot, dust and other solid particles. This signal quality relates, for example, to a mass flow of the medium through the measuring channel leading to the sensor chip, as well as, if necessary, to the reduction of a signal drift and the improvement of the signal-to-noise ratio. Here, the signal drift relates to the deviation, for example, of the mass flow of the medium in the sense of a change in the characteristic relationship between the actually occurring mass flow and the signal to be output in connection with the calibration during production. When the signal-to-noise ratio is ascertained, the sensor signals which are output in rapid time sequence are considered, while the characteristic or signal drift relates to a change in the mean value.
In conventional hot-film air-mass meters of the type described, a sensor carrier including a sensor chip attached to it or inserted into it generally protrudes into the measuring channel. For example, the sensor chip may be bonded into or bonded onto the sensor carrier. The sensor carrier may form a unit including, for example, a base plate made of metal, onto which an electronic system, an activation and evaluation circuit (for example, including a circuit substrate, in particular a circuit board) may be bonded. For example, the sensor carrier may be designed as an injection-molded plastic part of an electronic module. The sensor chip and the activation and evaluation circuit may, for example, be connected to one another by bonding connections. The electronic module created in this way may, for example, be bonded into a sensor housing and the entire plug-in sensor may be closed using covers.
German Patent Application No. DE 10 2010 020 264 A1 describes an air mass meter including a housing of plastic, which is electrically insulating, a flow channel being formed in the housing, and including a sensor element which is situated in the housing and which detects the air mass flowing in the flow channel. Situated in the housing are strip conductors which connect the sensor element to connecting pins. A part of the flow channel has electrostatically dissipative properties.
Despite the numerous advantages of the conventional methods for avoiding the contamination of the sensor element by, for example, dust particles, they still have potential for improvement. For example, the use of conductive plastics for flow-guiding components, which are connected to the ground line of the component, is a possible measure for avoiding contamination. This may reduce the static charge of the flow-guiding components, which increases the risk of contamination of the sensor element by dust particles. However, connecting the flow-guiding plastic components of the sensor housing to the carrier of the electronic components, such as a circuit board, requires an additional component made of conductive material, which results in increasing the cost of the sensor device. Furthermore, electrically charged particles are deposited in the flow-guiding part, which, although they are kept away from the sensor element, influence the flow prevailing in the measuring channel, which may in particular change the cross-section of the measuring channel.